1. Field of the Invention
The invention relates generally to the field of photography, and in particular to an improved photographic flash apparatus.
2. Description of the Prior Art
Electronic flash, as used in photography, is produced by an instantaneous electric discharge between two electrodes in a gas-filled glass tube. The tube is invariably mounted in a reflector to concentrate the light produced by the tube into a beam and redirect it toward a subject to be illuminated. The most efficient shape of the reflector is a parabola which concentrates the light into a beam that may have parallel, converging or diverging rays according to whether the tube is at the focal point of the parabola, in front of the focal point or behind it. Typically, the reflector is fitted with a transparent or translucent screen positioned substantially perpendicular to the beam at an open end of the reflector. The screen acts as a diffuser, softening the light and spreading it more evenly over the subject. However, this only covers the angle of view of a normal camera lens. When used with wide angle lenses, the flash unit may not illuminate the subject completely or evenly.
Various devices have been proposed to change the angle of coverage of the beam of light emitted from the reflector to conform with the selection of objective lens type, i.e. telephoto, wide angle, or normal. U.S. Pat. Nos. 4,176,930, issued Dec. 4, 1979, and No. 4,171,887, issued Oct. 23, 1979, each disclose a camera having a built-in flash unit with an auxiliary Fresnel lens or diffusion plate slidable to respective positions crosswise of the beam or removed from the beam in accordance the selection of different focal length objective lenses. The devices disclosed in these patents disadvantageously add to the size of the camera. U.S. Pat. No. 3,167,256, issued Jan. 26, 1965, discloses a flash unit adapted for use with several replaceable, removable lens plates each having a different optical property which is intended to match the film type or objective lens selected. With this device, it is possible for the user to lose one or more of the replaceable lens plates.